The invention relates to a process for the operation of a gas turbine plant with CO2 as the working medium.
Gas turbine plants with internal combustion and a quasi-closed circuit for CO2 as working medium represent examples of a very promising, environmentally friendly technique for energy recovery or conversion. Differing from conventional gas turbine plants for energy recovery, in which fossil fuels are used and large amounts of CO2 are released, gas turbine plants with a quasi-closed CO2 circuit make it possible to considerably reduce the CO2 emissions, and also emissions of nitrogen oxides, caused by the combustion of carbon-containing fuels in atmospheric oxygen. Thus in a known manner, the flue gases arising from the combustion process are cooled and by recirculation are supplied anew to the intake region of the gas turbine plant, with subsequent internal combustion. Such a recirculation, principally of CO2 which results from the combustion process, can only take place to the extent to which the atmospheric oxygen also present within the combustion process is used up. If the combustion process is fed with atmospheric oxygen, the flue gases arising in the combustion remain mixed with atmospheric nitrogen, so that the CO2 emission problem can however be only marginally reduced, and especially in this case the CO2 is mixed with nitrogen oxides in the resulting flue gases and can only be isolated from the circuit with greater difficulty.
In order to solve the said nitrogen problem with simultaneous environmentally friendly elimination of CO2, a gas turbine plant with a CO2 process has been proposed, as schematically shown in FIG. 5. The quasi-closed, CO2-charged gas turbine process shown in FIG. 5 has a combustion chamber 2 in which fossil fuel, for example natural gas (CH4) via the supply duct 6, is combusted with the exclusive addition of pure oxygen O2) through the supply duct 7. Since exclusively pure oxygen O2 is used as the oxidant, and no atmospheric oxygen is combusted as a result no nitrogen compounds enter into the further combustion cycle. The flue gases 21 emerging from the combustion chamber 2 drive a gas turbine 3, which is connected by a shaft 19 to a generator 5 for current production. The flue gases 21 expanding within the gas turbine 3 emerge as exhaust gases 20 from the gas turbine 3 and, via an external cooling heat exchanger 13, arrive directly in a compressor 18 in which they are compressed and, after exiting the compressor 18, are fed to a condenser 4. The compressor 18 is arranged on the common shaft 19 with the turbine 3 and also the generator 5 in the embodiment example shown in FIG. 5. Before the exhaust gases compressed by the compressor 18 enter the condenser 4, a recuperative withdrawal of heat takes place by means of a heat exchanger 14, so that the conditions fall below the condensation point of CO2 within the condenser 4 and the compressed and cooled CO2 passes into the liquid state. Water can be optionally branched off at the condensation point by means of a control valve 10. Uncondensed gas portions are removed from the circuit process via a control valve 9 from the condenser 4 which has a heat exchanger 12, and furthermore a partial flow of the liquefied CO2 is taken off via a control valve 8. The degree of charging, and thereby the power of the circuit process, can be controlled by the regulated tapping of CO2 from the circuit. From environmental standpoints, by separating the CO2 from the process by condensation, that state of aggregation of this gas is produced in which the CO2 arising can easily be disposed of under environmentally friendly conditions, especially as concerns the problem of greenhouse gases.
The liquefied main portion of CO2 which has not been branched off is compressed by means of a pump 1 and again supplied to the combustion chamber 2 in a correspondingly preheated and compressed form, via a duct 17 after passing through diverse recuperator stages 14, 15 and 16.
In order to be able to operate the above-described quasi-closed CO2 process with technically reasonable efficiencies, it is appropriate to ensure a complete condensation of the whole of the CO2. In order to be able to produce the liquid phase of CO2 in the condenser 4, pressure conditions of between 60 and 70 bar must prevail in the connecting duct between the compressor 18 and the condenser 4. Such a high output pressure at the beginning of condensation of the CO2 before entry into the condenser 4 leads, however, in the course of the compression by the pump 1 following the condenser 4, to an upper circuit pressure of 250-300 bar. Such a high pressure level is however not permissible within the combustion chamber, in view of the very high combustion temperatures which prevail there.
A further problem in the operation of the said gas turbine plants is represented by the extremely high heat capacity of highly pressurized CO2, which likewise rises with increasing pressure conditions. Thus even the three recuperatively acting heat exchangers 14, 15 and 16 shown in FIG. 5 are not sufficient for the CO2 flow, in order to heat the CO2 to a corresponding preheat temperature before entry into the combustion chamber.
The invention therefore has as its object to further develop a process for the operation of a gas turbine plant with CO2 as working medium, and also to develop a gas turbine plant of the said category related to this, so that the efficiency and the process parameters connected thereto are optimized within the quasi-closed CO2 circuit. In particular, measures are to be found which aid in preventing an overloading of the combustion chamber as regards its operating conditions.
According to the invention, a process for the operation of a gas turbine plant with CO2 as the working medium is disclosed. In at least one combustion chamber, hydrocarbons are combusted in a CO2 atmosphere enriched with oxygen to flue gases. The flue gases largely comprise CO2 and H2O and expand within a turbine stage following the at least one combustion chamber. The flue gases are then compressed in a compressor stage and also at least partially condensed in a following condenser such that at least a portion of the CO2 and H2O is liquefied and partially drawn off together with uncondensed flue gas components. A main portion not drawn off of liquid CO2 is compressed by means of a pump unit and preheated in at least one recuperator stage. The liquid CO2 is then supplied to the combustion chamber and developed such that the compressed main portion of CO2 is pre-expanded to a combustion pressure and, with the main portion CO2, is supplied for combustion to the combustion chamber.
By the measure according to the invention, of pre-expansion of the main portion CO2 compressed by the pump unit and typically at a pressure level between 250 and 300 bar after the pump unit, the high pressure level of the CO2 can be reduced to pressure values between 70 and 100 bar so that a safe and efficient operation of the combustion chamber is ensured.
The pressure reduction typically takes place using a turbine stage within the quasi-closed CO2 circuit, following the pump unit and effecting an efficient decompression of the CO2 gases before they enter the combustion chamber.
The additional turbine stage is preferably arranged in the CO2 circuit immediately upstream of the combustion chamber and effects the desired pressure reduction there. Between the pump unit and the previously sketched arrangement of the turbine stage, there is likewise preferably provided a multi-stage, for example three-stage, recuperator which preheats the CO2 gases compressed by the pump unit to a temperature desired for the combustion. Recuperator stages serve for this purpose and provide for a specific heat transfer from the expanded hot gases emerging from the turbine stage immediately following the combustion chamber, and/or from the hot CO2 gases emerging from the compressor stage before entry into the condenser, to the CO2 gases entering the further turbine stage for pre-expansion.